The engineers of the Wii Hombrew Channel, the launch application used for running unsigned software from the original Nintendo Wii's dashboard, have announced a new version of the HackMii crack that now also works on the Wii U (via Eurogamer):

For now, it only works on the Wii mode within the Wii U, not in full Wii U mode. In essence, the HBC runs on Wii U just as it does on Wii.

dhewg suggested the Wii hacker community may be able to find their way into the full Wii U mode, and thus get HBC running there, but it seems they are nowhere near that - yet.

“We can all try to hack the Wii U together from inside Wii mode,” he said.

Nintendo won't be happy with this latest development. It maintains that the installation of the Homebrew Channel voids warranty as it is considered unauthorised software.

That unsigned code is already running on the Wii U at all is quite significant.

Several prominent Wii U hackers have also already reportedly been hinting at more a complete Wii U security exploit, but any related cracks or software are yet to be publicly released.

I'm pretty sure the first exploits for the Wii were done from the gamecube mode.

Nope, Twilight Hack was the first, and it was a modified save game for the Wii version of Twilight Princess. It's actually harder to hack the Wii via the GameCube mode, since it basically locks out access to the extra Wii hardware. WiiU is exactly the same with regards to Wii backwards compatibility (i.e. it locks out WiiU functions when running in Wii mode), so there's no reason to cut backwards compatibility.

Also, this isn't the first Wii exploit known to work on WiiU, SmashStack still works.

3DS is still not cracked yet, it has been out for ages too. Nintendo are doing a pretty good job this time round of locking down their hardware. Playstation 3 still holds the crown for toughest nut I think.

The SmashStack exploit was demonstrated to be working the day the Wii U launched in the US. This isn't a new hack, this is just a new version of the Homebrew channel for people to install on the Wii U (while running in Wii mode). It's also still a lot more limited than regular Wii hacks, since I don't think you can install custom IOSs. That means no region unlocking (boo!), but also no piracy (good!).